The purpose of this project is to study the phenomenon of epithelial repair following ionizing radiation injury. This will be done by determining epithelial cell population kinetics. More specifically, it is intended to study the clustering of mitotic cells in chemically protected mouse tongue. This clustering was observed by us and previously suspected by other workers, Cammeron and Schewing. The observations made in our prior studies indicated that irradiated chemically protected epithelium did not begin replacement of lost cells from the generative compartment, basal cell layer, to the differentiated compartment, Malphigian layer, did not merely depend upon mitosis in basal cells but rather upon mitosis in cells spatially related in clusters. Apparently this phenomenon becomes pronounced during repair following irradiation injury since we have detected this clustering at a much less degree in nonirradiated epithelium. For the purposes of our own experiments we have used the word symphokinesis to indicate this phenomenon.